Resistance changing element, semiconductor device, and method for forming resistance change element

ABSTRACT

A resistance changing element according to the present invention comprises a first electrode ( 101 ) and a second electrode ( 103 ); and an ion conducting layer ( 102 ) that is formed between the first electrode ( 101 ) and the second electrode ( 103 ) and that contains at least oxygen and carbon.

TECHNICAL FIELD

The present invention relates to a semiconductor device and a methodthat manufactures the same. In particular, the present invention relatesto a semiconductor device that is provided with memory having resistancechanging type non-volatile elements (hereinafter referred to asresistance changing elements) formed in a multi-layered wiring layer anda field programmable gate array (FPGA); a resistance changing element;and a manufacturing method that forms the same.

BACKGROUND ART

Semiconductor devices including silicon devices have been developed forhigh integration and low power consumption by miniaturization of thedevices according to a scaling law known as the Moors law that says that“the number of transistors quadruples every three years.” In recentyears, the gate lengths of MOS FETs (Metal Oxide Semiconductor FieldEffect Transistors) have become 20 nm or less. Because of steep pricerises for lithography processes and physical limitations of devicedimensions, there has been demand for improved device performance notbased on the traditional scale laws.

The steep price rises of lithography processes have resulted from thesteep price increase of manufacturing apparatus and mask set. On theother hand, the physical limitations of the device dimensions haveresulted from the limitations of operation and dimensional fluctuations.

In recent years, rewritable programmable logic devices called FPGAs,that lie between gate arrays and standard cells, have been developed.FPGAs allow the user to configure circuits after their chips aremanufactured. FPGAs are provided with resistance changing elements intheir multi-layered wiring layer such that the user himself or herselfcan configure his or her desired circuit. FPGAs have resistance changingelements in their multi-layered wiring layer such that the user can makeelectric wire connections in a predetermined manner. Semiconductordevices provided with such an FPGA have improved the freedom degree ofcircuit design. Resistance changing elements include ReRAM (ResistanceRAM (Random Access Memory)) made of a transition metal oxide andNanoBridge (registered trademark of NEC) made of an ion conductor. Anion conductor is a solid state material in which ions can freely migrateby applying an electric field or the like.

As a resistance changing element that is likely to improve the freedomdegree of circuit design, a switching element that uses the migration ofmetal ions and electrochemical reactions in an ion conductor isdisclosed in literature (Shunichi KAERIYAMA et al., “A NonvolatileProgrammable Solid-Electrolyte Nanometer Switch,” IEEE Journal ofSolid-State Circuits, Vol. 40, No. 1, pp. 168-176, January 2005).

The switching element disclosed in this literature is composed of threelayers that are an ion conducting layer, a first electrode, and a secondelectrode, the first and second electrodes being in contact with twoprimary surfaces of the ion conducting layer. Among these layers, thefirst electrode serves to supply metal ions to the ion conducting layer.In contrast, the second electrode does not supply metal ions to the ionconducting layer.

Here, operation of this switching element will be described in brief.When the first electrode is grounded and then a negative voltage isapplied to the second electrode, the metal of the first electrodechanges to metal ions and dissolve into the ion conducting layer.Thereafter, the metal ions in the ion conducting layer deposit as themetal in the ion conducting layer and the deposited metal forms metalbridges that electrically connect the first electrode and the secondelectrode. When the metal bridges electrically connect the firstelectrode and the second electrode, the switching element enters the ONstate.

In contrast, while the switching element is in the ON state, when thefirst electrode is grounded and then a positive voltage is applied tothe second electrode, part of the metal bridges is cut. As a result, theelectric connection between the first electrode and the second electrodeis cut and thereby the switching element enters the OFF state. Beforethe electric connection is completely cut, electric characteristicschange, for example, the resistance between the first electrode and thesecond electrode becomes large and the capacitance therebetween changesand finally, the electric connection is cut. To change the state of theswitching element from the OFF state to the ON state, the firstelectrode is grounded and then a negative voltage is applied to thesecond electrode.

In addition, the foregoing literature discloses a structure andoperation of a two-terminal type switching element that has twoelectrodes faced each other through an ion conductor and that isoperated by controlling the conducting state therebetween.

Such a switching element is smaller and has a lower on-resistance than asemiconductor switch such as an MOS FET. Thus, it is thought that such aswitching element has a potential to be applied to programmable logicdevices. In addition, since this switching element can maintain theconducting state (ON or OFF state of the element) without the need toapply a voltage, it may be used for a nonvolatile memory element.

For example, a plurality of memory cells as basic elements eachcontaining one selector element and one switching element composed offor example transistors are arranged in the vertical and horizontaldirections. When the memory cells are arranged in such a manner, any oneof a plurality of memory cells can be selected using word wires and bitwires. As a result, a nonvolatile memory that senses the conductingstate of a switching element of the selected memory cell and readsinformation of “1” or “0” based on the ON or OFF state of the switchingelement can be realized.

SUMMARY OF INVENTION

Since there has been demand in recent years for semiconductor deviceshaving low power consumption, the power supply voltages of CMOS deviceshave been decreased. Likewise, demand has arisen for operation voltagesof resistance changing elements that are mounted on CMOS devices to beused for operation voltages of transistors mounted on regular logiccircuits. However, to operate resistance changing elements at theoperation voltages of transistors, an electrical initial activationtreatment called forming is required. Generally, voltage necessary forforming (hereinafter referred to as the forming voltage) is at least 4 Vor higher.

For example, for an element that uses copper and an ion conducting layeras a resistance changing element (for example, Nanobridge (registeredtrademark)), the use of Cu₂S, TaO, TaSiO, or the like for the ionconducting layer has been proposed. When a Cu₂S material is used for theion conducting layer, the operation voltage becomes 0.2 V or below andthereby the operation voltage adversely becomes too low. On the otherhand, when a TaO material is used for the ion conducting layer, theinitial set voltage for the foregoing forming voltage becomes 4 V orabove and thereby the initial set voltage adversely becomes too high.Thus, to realize a CMOS-logic compatible operation, the requirement isthat the forming voltage and set voltage to be set to 3.3 V or below,which is the operation voltage of ordinary I/O transistors.

As a technique that lowers the forming voltage, the film thickness ofthe ion conducting layer is decreased so as to increase the effectiveelectric field. However, in this technique, when an ordinary material isformed as a film, if a reset voltage (reverse bias) is applied, adielectric breakdown voltage adversely becomes low and thereby theforming voltage cannot be lowered. The dielectric breakdown voltage is avoltage at which a dielectric breakdown takes place when voltage that isapplied to a film under measurement is increased.

An exemplary object of the invention is to provide a resistance changingelement that can operate at low voltages while keeping reliability, asemiconductor device, and a method that forms the resistance changingelement.

A resistance changing element according to an exemplary aspect of theinvention includes a first electrode and a second electrode; and an ionconducting layer that is formed between the first electrode and thesecond electrode and that contains at least oxygen and carbon.

A semiconductor device according to an exemplary aspect of the inventionincludes a resistance changing element according to the presentinvention.

A method that forms a resistance changing element, according to anexemplary aspect of the invention includes forming an insulation film byplasma CVD process with a gas in which a gas of an organic silicacompound having a skeleton of at least silicon and oxygen is dilutedwith an inertia gas; and forming a second electrode on the insulationfilm.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a sectional view schematically showing an example of astructure of a semiconductor device according to a first embodiment.

FIG. 2 is a block diagram showing an example of a structure of a plasmaCVD apparatus.

FIG. 3 is a sectional view schematically showing an example of anotherstructure of the semiconductor device according to the first embodiment.

FIG. 4 is a sectional view showing an example of a structure of asemiconductor device according to a second embodiment.

FIG. 5A is a sectional view describing a method for forming thesemiconductor device according to the second embodiment.

FIG. 5B is a sectional view describing the method for forming thesemiconductor device according to the second embodiment.

FIG. 5C is a sectional view describing the method for forming thesemiconductor device according to the second embodiment.

FIG. 6 is a sectional view showing an example of a structure of asemiconductor device according to a third embodiment.

FIG. 7A is a sectional view describing a method for manufacturing thesemiconductor device shown in FIG. 6.

FIG. 7B is a sectional view describing the method for manufacturing thesemiconductor device shown in FIG. 6.

FIG. 7C is a sectional view describing the method for manufacturing thesemiconductor device shown in FIG. 6.

FIG. 8A is a sectional view describing the method for manufacturing thesemiconductor device shown in FIG. 6.

FIG. 8B is a sectional view describing the method for manufacturing thesemiconductor device shown in FIG. 6.

FIG. 8C is a sectional view describing the method for manufacturing thesemiconductor device shown in FIG. 6.

FIG. 9A is a sectional view describing the method for manufacturing thesemiconductor device shown in FIG. 6.

FIG. 9B is a sectional view describing the method for manufacturing thesemiconductor device shown in FIG. 6.

FIG. 10A is a sectional view describing the method for manufacturing thesemiconductor device shown in FIG. 6.

FIG. 10B is a sectional view describing the method for manufacturing thesemiconductor device shown in FIG. 6.

FIG. 11 is a graph showing I-V characteristics of a resistance changingelement according to working example 1.

FIG. 12 is a graph showing a dielectric breakdown voltage of a porousfilm according to working example 1.

FIG. 13A is a semiconductor device showing principal sections of anexample of a structure of a memory device using the resistance changingelements according to working example 1.

FIG. 13B is a semiconductor device showing an example of a structure ofa memory cell of a memory cell array shown in FIG. 13A.

FIG. 14 is a graph showing a distribution of resistance values of thememory cell array shown in FIG. 13A.

DESCRIPTION OF EMBODIMENTS

A resistance changing element according to an embodiment of the presentinvention has a structure in which a first electrode, an insulation filmcontaining at least oxygen and carbon, and a second electrode aresuccessively arranged.

Since the resistance changing element according to this embodiment usesa porous insulation film containing at least oxygen and carbon as an ionconducting layer, bridges can be easily formed by metal ions in the ionconducting layer and thereby the forming voltage can be lowered.

An intensive study for voltages at which metal bridges were formed in anion conducting layer and for materials of ion conducting layer conductedby the inventors of the present invention revealed that the lower thefilm density of the ion conducting layer, the lower the forming voltageat which initial bridges are formed.

In other words, to change the resistance changing element to the ONstate (low resistance state), metal bridges need to be formed in the ionconducting layer. However, since holes formed in the film of the ionconducting layer contribute to decreasing the volume change for themetal bridges, the substance migration amount becomes small and therebythe forming voltage can be lowered.

As a technique that decreases the density of the insulation film thatbecomes the ion conducting layer, when a film having a skeleton of SiOis formed, it is effective to add carbon (C) to the film. When an alkylchain such as a methyl group or an ethyl group or an alkane chain suchas a vinyl group is introduced in the insulation film, the density ofthe insulation film can be decreased.

Alternatively, when holes of several nm or less are intentionallyintroduced in the insulation film, the density of the insulation filmmay be decreased and thereby the forming voltage may be lowered.However, since the distribution of hole diameters corresponds to thedistribution of the forming voltages, if an insulation film having amulti-peak distribution of hold diameters is used for the ion conductinglayer, the forming voltages adversely fluctuate. Thus, to decrease thefluctuations of the operation voltages of the resistance changingelement, it is preferred that the insulation film have a single peakdistribution of hole diameters.

As another effect of this embodiment in which at least either a SiO filmor a porous film is used for the ion conducting layer, the relativepermittivity of the ion conducting layer can be decreased. As a result,since a graph that represents the relationship between the formation ofmetal bridges and applied voltages becomes steep, the disturbcharacteristics in the operation voltage range can be improved.

In addition, the inventors of the present invention revealed that whenporous film according to this embodiment is used for an ion conductinglayer, the dielectric breakdown voltage corresponding to the resetvoltage (reverse bias voltage) can be raised. This means that when aporous film is used as a material subject to the formation of bridgesfor an ion conducting layer, damages (defects) formed in the ionconducting layer upon forming are decreased and thereby reliability ofthe insulation can be improved.

On the hand, if the ion conducting layer according to this embodiment isa film that contains oxygen, an oxide film of a valve metal (forexample, a titanium oxide film) may be formed on a copper electrode. Inthis case, the oxide film of the valve metal prevents the lower copperlayer from being oxidized. This means that since the standard freeenergy of an oxide of a valve metal such as titanium (Ti) or aluminum(Al) is greater than that of copper in the minus direction, the valvemetal absorbs oxygen generated in the ion conducting layer.

A valve metal is a metal that prevents copper from being oxidized.Examples of valve metals are tantalum, niobium, hafnium, zirconium,zinc, tungsten, bismuth, and antimony besides Ti and Al. In thisembodiment, a valve metal is used to prevent copper from being oxidized.However, a valve metal may be used to prevent elements other than copperfrom being oxidized.

In addition, experiments that the inventors conducted to realize thepresent invention also revealed that when a porous film is formed, ifthe film contains a six-membered ring-shaped siloxane structure or aneight-membered ring-shaped siloxane structure having a skeleton ofsilicon and oxygen, copper ions can easily enter and exit from the ringsof the film and thereby the ON/OFF operations of the resistance changingelement can be easily performed.

Alternatively, a porous film may be formed by using a ring-shapedorganic silica compound in which at least one unsaturated hydrocarbongroup is bonded to its side chain. In this case, while the material ofthe insulation film is prevented from dissolving and reacting, it can begrown. Thus, when a porous film is formed on a copper electrode, whilecopper is prevented from being oxidized, the porous film can be formed.In addition, while a ring-shaped siloxane skeleton of the material iskept, an insulation film can be formed. Thus, an insulation film thatcontains a six-membered ring-shaped siloxane structure or aneight-membered ring-shaped siloxane structure having the foregoingskeleton of silicon and oxygen can be suitably obtained and therebycarbon can be left in the porous film.

Alternatively, when a mixed gas of a gas of a ring-shaped silicacompound in which at least one unsaturated hydrocarbon group is bondedto its side chain and an inertia gas is supplied to the first electrode,a porous film can be formed on the first electrode by plasma CVD(Chemical Vapor Deposition) process.

A resistance changing element according to another embodiment of thepresent invention has a structure in which ruthenium (Ru) as a firstelectrode, TiO₂ as a resistance changing layer, a porous film as abuffer layer, and Ru as a second electrode are successively formed.

In the following, the embodiments of the present invention will bedescribed in detail.

First Embodiment

Here, with reference to the accompanying drawings, a semiconductordevice according to a first embodiment of the present invention will bedescribed. FIG. 1 is a sectional view schematically showing an exampleof a structure of the semiconductor device according to this embodiment.

As shown in FIG. 1, the semiconductor device according to thisembodiment has first electrode 101, porous film 102, and secondelectrode 103. Formed successively on first electrode 101 are porousfilm 102 and second electrode 103. First electrode 101 is composed ofmetals whose main component is copper. Porous film 102 contains silicon(Si), oxygen (O), and carbon (C) as constituent elements.

Here, first electrode 101 and second electrode 103 of the semiconductordevice according to this embodiment will be described. First electrode101 is composed of metals containing copper. First electrode 101 servesto supply copper ions to the ion conducting layer. First electrode 101may contain impurities Al, Ti, tin (Sn), and so forth as well as copper.The materials of second electrode 103 are preferably Ru, nickel (Ni),titanium nitride (TiN), or platinum (Pt).

Next, the ion conducting layer of the semiconductor device according tothis embodiment will be described. According to this embodiment, porousfilm 102 serves as an ion conducting layer. The main components of theion conducting layer are preferably Si, O, and C. When porous film 102has a ring-shaped siloxane structure composed of Si and O, ions can beeasily conducted in the film. In addition, the ion conducting layer ispreferably a film composed of Si, O, and C as constituent elements. Asone reason to do so, when the insulation film as the ion conductinglayer contains C, the relative permittivity of the ion conducting layercan be decreased. As a result, since the graph that represents therelationship between the formation of metal bridges and applied voltagesbecomes steep, the disturb characteristics in the operation voltagerange can be improved.

In addition, the ion conducting layer preferably has a relativepermittivity of 2.5 to 3.5 and a single peak distribution of holediameters. Generally, when the ion conducting layer is an insulationfilm having a skeleton of SiOC, if the insulation film has a relativepermittivity of 3.5 or greater, the film density adversely becomes largeand thereby becomes unsuitable for an ion conducting layer. In contrast,if the relative permittivity of the insulation film is maintained at 2.5or less, the water absorption coefficient increases and thereby the leakcurrent increases. Moreover, the reason why the insulation filmpreferably has a single peak distribution of hole diameters is that ifhole diameters largely fluctuate, the foaming voltages also fluctuate.

The composition ratios of C/Si and O/Si of the porous film as the ionconducting layer are preferably in the range from 0.4 to 3.5 and in therange from 0.5 to 1.5, respectively.

The semiconductor device according to this embodiment is a resistancechanging element that uses the migration of metal ions andelectrochemical reactions in an ion conductor. The first electrodeserves to supply copper ions to the ion conducting layer. The ON/OFFcontrol for the resistance changing element is performed by causingvoltage to be applied or a current to flow thereto. Specifically, theON/OFF control for the resistance changing element is performed by usingelectro-migration of copper ions from the first electrode to the ionconducting layer.

Next, a method that forms a porous film used for the ion conductinglayer will be described. The method that forms a porous film includesthe step that forms a porous film on a first electrode using a gas inwhich a gas of an organic silica compound having a skeleton of at leastsilicon and oxygen is diluted with an inertia gas.

The organic silica compound is preferably a ring-shaped organic silicacompound that has a skeleton of silicon and oxygen and in which at leastone unsaturated hydrocarbon group is bonded to the side chain.

Specifically, the organic silica compound preferably has a structureexpressed by Chemical Formula 1 or Chemical Formula 2, where R1 and R2each is any one from among hydrogen, a methyl group, an ethyl group, apropyl group, an isopropyl group, and a vinyl group.

Chemical Formula 3 expresses the structure of an organic silica compoundin which R1 is a methyl group and R2 is a vinyl group in the structureexpressed by Chemical Formula 1.

Chemical Formula 4 expresses the structure of an organic silica compoundin which R1 is an ethyl group and R2 is a vinyl group in the structureexpressed by Chemical Formula 1.

Chemical Formula 5 expresses the structure of an organic silica compoundin which R1 is an isopropyl group and R2 is a vinyl group in thestructure expressed by Chemical Formula 1.

Chemical Formula 6 expresses the structure of an organic silica compoundin which R1 is a methyl group and R2 is hydrogen in the structureexpressed by Chemical Formula 1.

Chemical Formula 7 expresses the structure of an organic silica compoundin which R1 is a methyl group and R2 is a vinyl group in the structureexpressed by Chemical Formula 2.

Chemical Formula 8 expresses the structure of an organic silica compoundin which R1 is a methyl group and R2 is hydrogen in the structureexpressed by Chemical Formula 2.

Chemical Formula 9 expresses the structure of an organic silica compoundin which R1 is a vinyl group and R2 is hydrogen in the structureexpressed by Chemical Formula 2.

Chemical Formula 1 is a six-membered ring because the total number ofSi's and O's is 6.

Likewise, each of Chemical Formula 3 to Chemical Formula 6 is asix-membered ring. On the other hand, Chemical Formula 2 is aneight-membered ring because the total number of Si's and O's is 8.Likewise, each of Chemical Formula 7 to Chemical Formula 9 is asix-membered ring. When any one of the materials expressed by ChemicalFormula 1 to Chemical Formula 9 is used, the composition ratio of 0/Siof the formed insulation film is around 1 that ranges from 0.5 to 1.5.

Although this embodiment does not present a specific example of anorganic silica compound in which R1 or R2 of Chemical Formula 1 orChemical Formula 2 is a propyl group, experimental results for thematerials expressed by Chemical Formula 1 to Chemical Formula 9 predictthat they can be used for materials from which a porous film accordingto this embodiment is formed even if R1 or R2 is a propyl group.

Next, the method that forms a porous film according to this embodimentwill be described in detail. In this example, the case in which a porousfilm is formed from an organic siloxane material and an inertia carriergas will be described.

FIG. 2 is a block diagram showing an example of a structure of a plasmaCVD apparatus that is used to form a porous film by the method accordingto this embodiment.

As shown in FIG. 2, the plasma CVD apparatus has reaction chamber 210,gas supply section 220, vacuum pump 230, and radio frequency (RF) powersupply 240. Gas supply section 220 is connected to reaction chamber 210through gas supply pipe 222. Vacuum pump 230 is connected to reactionchamber 10 through gas exhaust pipe 236. Gas exhaust pipe 236 isprovided with valve 232 and cooling trap 234. Connected to RF powersupply 240 is reaction chamber 10 through RF cable 244. RF cable 244 isprovided with matching box 242.

Located in reaction chamber 210 are substrate heating section 203 andshower head 205 that face each other. Connected to substrate heatingsection 203 is ground wire 207. Substrate heating section 203 is alsoprovided with a heater (not shown). Substrate heating section 203 holdswork piece 201 such as a semiconductor substrate and heats work piece201. Connected to shower head 205 is gas supply pipe 222. Shower head205 serves as a gas spray section that sprays gas supplied through gassupply pipe 222 to work piece 201. Connected to shower head 205 is alsoRF cable 244.

While gas supply section 220 supplies a material gas to shower head 205through gas supply pipe 222, matching box 242 located in the middle ofRF cable 244 supplies a radio frequency power at a predeterminedfrequency generated by RF power supply 240 to shower head 205 so as toionize the gas in the space between substrate heating section 203 andshower head 205 as plasma.

Gas supply section 220 is provided with material supply tankscorresponding to the number of kinds of ring-shaped organic siloxanematerial gases for use and an admixture gas tank (may be referred to asgas supply tank). In addition, gas supply section 220 has mixer 219 thatmixes gases supplied from the individual tanks. FIG. 2 shows the case inwhich gas supply section 220 is provided with material supply tank 211that is filled with one ring-shaped organic siloxane material andadmixture gas supply tank 215. Admixture gas may be omitted.

One end of gas supply pipe 222 is connected to mixer 219. Materialsupply tank 211 is connected to mixer 219 through pipe 212, whereasadmixture gas supply tank 215 is connected to mixer 219 through pipe216.

Pipe 212 is provided with flow rate control section 253 and evaporator260 that are located between material supply tank 211 and mixer 219.Flow rate control section 253 has two valves 251 a and 251 b and flowrate controller 252 located therebetween. Supplied to evaporator 260 isan inert gas as a carrier gas. In this example, helium (He) gas as aninertia gas is supplied to evaporator 260. Evaporator 260 evaporates aliquid material supplied from material supply tank 211 and supplies thematerial gas along with the carrier gas to mixer 219.

Pipe 216 is provided with flow rate control section 257 located betweenadmixture gas supply tank 215 and mixer 219. Flow rate control section253 has two valves 255 a and 255 b and flow rate controller 256 locatedtherebetween.

As shown in FIG. 2, connected in the middle of gas supply pipe 222 iscleaning gas supply pipe 228. Cleaning gas supply pipe 228 is providedwith flow rate controller 224 and valve 226. Drain pipe 238 branchesfrom gas exhaust pipe 236 that is located between valve 232 and coolingtrap 234.

Heaters (not shown) are preferably located around pipes 212 and pipe 216and around gas supply pipe 222 so as to heat pipes 212 and 216 and gassupply pipe 222 and prevent each gas that is being supplied from beingliquidified. Likewise, a heater is preferably located around reactionchamber 210 so as to heat reaction chamber 210 and prevent gas that isbeing supplied to reaction chamber 210 from being liquidified before itsmolecules are exited.

Next, a procedure of the method that forms a porous film using plasmaCVD apparatus 50 will be described.

After work piece 201 is placed on substrate heating section 203, vacuumpump 230 is operated and valve 232 is opened such that the initialvacuum degree of reaction chamber 210 is decreased to several Torr.Moisture contained in the gas exhausted from reaction chamber 210 isremoved by cooling trap 234. Substrate heating section 203 heats workpiece 201 such that the surface temperature of work piece 201 lies in apredetermined temperature range. Note that 1 Torr is around 133 Pa andTorr may be partly used as the unit of pressure.

Thereafter, a mixed gas of the material gas (in this example, an organicsiloxane gas), the carrier gas, and the admixture gas is supplied frommixer 219 to reaction chamber 210 through gas supply pipe 222. Inaddition, RF power supply 240 and matching box 242 are operated so as tosupply RF power at the predetermined frequency to reaction chamber 210.

When the material gas is supplied to reaction chamber 210, flow ratecontrol section 253 controls the flow rate of the organic siloxane gasand flow rate control section 257 controls the flow rate of the carriergas such that mixer 219 generates a mixed gas with a predeterminedcomposition. The generated mixed gas is supplied to reaction chamber210. The partial pressure of the material gas in reaction chamber 210 ispreferably maintained in the range from 0.1 to 3 Torr. In addition, whena porous film is formed, the ambient pressure in reaction chamber 210 ispreferably maintained in the range from 1 to 6 Torr by controllingvacuum pump 230. At this point, the partial pressure of the material gasis preferably 0.3 Torr or below so as to obtain an insulation filmhaving a low relative permittivity.

Work piece 201 is preferably maintained by substrate heating section 203at a temperature in the range of 100 to 400 degrees Celsius, morepreferably in the range from 200 to 350 degrees Celsius.

In such process conditions, molecules of the organic siloxane materialas a material gas are excited and activated molecules as plasma reachthe front surface of work piece 201 and thereby a porous film is formedthereon. If the molecules of the material gas have an unsaturatedbonding group, when the molecules are exited and activated as plasma andreach the front surface of work piece 201, since they receive heat fromsubstrate heating section 203, the unsaturated bonding group in eachmolecule is caused to open its ring and the thermal polymerizationreaction progresses between the molecules such that a porous film isformed on work piece 201.

When a silicon oxide film is formed as an insulation film, oxidation gasis normally used. According to this embodiment, when a porous film isformed as an insulation film, since an oxidation gas is used neither forthe admixture gas, nor for the carrier gas, even if a metal that issubject to oxidation (for example, copper) is used for the material offirst electrode 101, the metal can be prevented from being oxidized.

FIG. 2 shows the case in which the number of gas supply tanks 211 isone. Alternatively, a plurality of material supply tanks may be providedso as to form a porous film from a plurality of kinds of material gases.When a porous film is formed from a plurality of kinds of materialgases, by changing the flow rate ratio of a six-membered ring-shapedsiloxane structure material gas and an eight-membered ring-shapedsiloxane structure material gas, the ion conductivity of the porous filmcan be adjusted to a desired value. The number of kinds of materialgases for use is not limited to two, but may be three or more.

Alternatively, a program that codes the instructions for the procedureof the method that forms the foregoing porous film for substrate heatingsection 203, vacuum pump 230, flow rate control sections 253 and 257, RFpower supply 240, matching box 242, and valve 226 and 236 may beprepared and a microcomputer may be caused to execute a processaccording to the program so as to control the plasma CVD apparatus andform the porous film.

Here, a method that prevents the front surface of an electrode on whicha porous film is formed from being oxidized will be described.

When a porous film is formed according to the foregoing method, sincethe organic siloxane material contains oxygen, while the porous film isbeing formed, oxygen plasma may be generated from the partly decomposedmaterial. In this case, if first electrode 101 shown in FIG. 1 containscopper, oxygen plasma generated while the film is being formed oxidizesthe front surface of the electrode and thereby a problem in whichdesired resistance changing characteristics cannot be obtained in theion conducting layer may occur.

To solve the problem in which the front surface of the first electrodeis adversely oxidized, after the first electrode is formed before theporous film is formed, it is preferable to form a valve metal film (notshown) and oxidize the valve metal while the porous film is beingformed.

In another structure of this embodiment, an oxide film of a valve metalsuch as titanium or aluminum is formed in contact with the firstelectrode. When the valve metal is titanium, its oxide film is atitanium oxide film; when the valve metal is aluminum, its oxide film isan aluminum oxide film. A valve metal is a metal that is subject topassivation.

Since an oxide film of a valve metal has negative large standard freeenergy compared to copper, the oxide film absorbs oxygen that isgenerated while the ion conducting layer is being formed and therebyprevents copper from being oxidized.

Although copper wires are generally formed by electroplating process,impurities or the like contained in a plating solution contain a traceamount of oxygen. Oxygen that resides in copper causes copper bridgesthat are formed upon switching to highly fluctuate. A valve metal alsoserves to absorb oxygen that is generated from copper wires formed inthe lower layer. The film thickness of the valve metal is preferably 4nm or less.

If an oxide film of a valve metal such as a titanium oxide film or analuminum oxide film has been formed between the first electrode and theion conducting layer, when the device is switched from the lowresistance state (ON state) to the high resistance state (OFF state),bridges of copper ions that pierce the ion conducting layer and theoxide film of the valve metal are broken by the oxide film of the valvemetal and thereby the copper bridges in the oxide film of the valvemetal are preferentially collected. As a result, an electric field isapplied to the inside of the oxide film of the valve metal and therebythe copper ions can be easily collected from the ion conducting layer.Thus, the switching characteristics of the resistance changing elementcan be improved.

Although the method that forms a titanium oxide film and an aluminumoxide film that are examples of oxide films of valve metals is notlimited, they can be formed for example by the following steps. First, atitanium film or an aluminum film is formed on the first electrode.Thereafter, an ion conducting layer is formed on the titanium film oraluminum film by the sputtering process. When the ion conducting layeris formed by the sputtering process while oxygen gas is being supplied,the titanium film or aluminum film is oxidized as a titanium oxide filmor aluminum oxide film.

The film thickness of the titanium oxide film is preferably in the rangefrom 1 to 3 nm. Since metal bridges formed in and collected from the ionconducting layer are controlled by an electric field, when the filmthickness of the titanium oxide film is 3 nm or less, the voltagenecessary for switching can be lowered.

Next, a surface treatment for porous film 102 of the resistance changingelement shown in FIG. 1 will be described. This treatment is performedby radiating plasma of an inertia gas to the front surface of porousfilm 102 after porous film 102 is formed before second electrode 103 isformed.

In this example, the plasma CVD apparatus shown in FIG. 2 is used. Afterporous film 102 is formed in reaction chamber 210, He gas is supplied toreaction chamber 210 and then the internal pressure of reaction chamber210 is set in the range from around 1 to 6 Torr. Thereafter, RF power issupplied to shower head 205 so as to generate He plasma, remove carbonfrom the front surface of the porous film, and form SiO₂ thereon.

FIG. 3 is a sectional view showing an example of another structure ofthe semiconductor device according to this embodiment. As shown in FIG.3, modified layer 117 is formed on the front surface of porous film 102.When the front surface of porous film 102 is treated with He plasma,carbon is removed from the front surface of the porous film and thenmodified layer 117 is formed on the front surface of porous film 102,the carbon content of modified layer 117 being lower than that of porousfilm 102. In this case, SiO₂ that is a high density film is formed onthe porous film such that SiO₂ is in contact with the second electrode.When an oxide film of a valve metal is formed between the firstelectrode and the porous film, a structure in which the oxide film ofthe valve metal, the porous film, the SiO₂ film, and the secondelectrode are successively formed is obtained viewed from the firstelectrode.

Since the conductivity of copper ions in the structure decreases in theorder of the oxide film of the valve metal, the porous film, and theSi0₂ film, when the low resistance state (ON state) is switched to thehigh resistance state (OFF state) in the structure where they aresuccessively formed, copper bridges that are deposits of copper ionsformed through the ion conducting layer, which contains the porous filmand its upper SiO₂ film, and the oxide film of the valve metal arepreferentially collected. As a result, an electric field is applied tothe inside of the oxide film of the valve metal and the inside of theporous film and thereby copper ions can be easily collected.Consequently, the switching characteristics of the resistance changingelement can be improved.

Second Embodiment

Next, the structure of a resistance changing element according to asecond embodiment will be described. FIG. 4 is a sectional view showingan example of a structure of the resistance changing element accordingto this embodiment.

As shown in FIG. 4, the resistance changing element according to thisembodiment has first electrode 110, titanium oxide film 112, ionconducting layer 113, and second electrode 114. Formed successively onfirst electrode 110 are titanium oxide film 112, ion conducting layer113, and second electrode 114.

Next, with reference to FIG. 5A to FIG. 5C, a method that manufacturesthe resistance changing element according to this embodiment will bedescribed in detail. It should be noted that the present invention isnot limited to the following embodiment.

In the following, with reference to FIG. 5, an example of the method formanufacturing the resistance changing element according to the presentinvention will be described in brief. First, titanium film 112 a isformed on first electrode 110 that is composed of metals whose maincomponent is copper (FIG. 5A). Thereafter, ion conducting layer 113 isformed on titanium film 112 a and then titanium film 112 a is oxidizedso as to form titanium oxide film 112 (FIG. 5B). Thereafter, secondelectrode 114 is formed on ion conducting layer 113 (FIG. 5C).

The resistance changing element manufactured according to thisembodiment has a structure in which titanium oxide film 112 and ionconducting layer 113 are formed between second electrode 114 that is anupper electrode and first electrode 110 that is a lower electrode. Onthe other hand, the resistance changing element has a laminate structurein which first electrode 110, titanium oxide film 112, ion conductinglayer 113, and second electrode 114 are successively formed. Firstelectrode 110 is a metal film that contains copper, whereas ionconducting layer 113 is a porous film whose main components are Si, C,and O.

Next, a specific example of the method for forming the resistancechanging element shown in FIG. 4 will be described in detail. In thisexample, it is assumed that a silicon wafer is used as a work piece.

The material of first electrode 110 is copper and first electrode 110 isformed on a substrate (not shown) by sputtering process or byelectroplating process. The material of first electrode 110 may containimpurities of Al, Sn, Ti, and so forth as well as copper.

Thereafter, as shown in FIG. 5A, titanium film 112 a is formed on firstelectrode 110. Titanium film 112 a is formed by DC (Direct Current)sputtering process. When titanium film is formed on an eight-inchsilicon wafer, it can be obtained at a growth speed of 22 nm/min underthe conditions in which the inner pressure of the reaction chamber is0.35 [Pa], the flow rate of Ar is 40 sccm, the temperature of thesubstrate is at room temperature, and the sputtering power is 0.2 kW.The film thickness of titanium that is deposited is preferably 2 nm orless.

Next, a step that forms ion conducting layer 113 on titanium film 112 awill be described. A porous film whose main components are Si, C, and Ocan be formed by plasma CVD process using a gas of an organic siloxanematerial having the structure expressed by Chemical Formula 2 orChemical Formula 3. In the following, a case in which the plasma CVDapparatus shown in FIG. 2 is used will be described.

Alternatively an SiOCH film such as “Aurora (registered trademark),”“Aurora-ULK (registered trademark),” or “Black Diamond (registeredtrademark)” formed by plasma CVD process may be used for ion conductinglayer 113. However, when copper wires are used in a lower layer, theupper surface of copper needs to be coated with a valve metal so as toprevent copper from being oxidized.

For example, as the material of an organic siloxane, the material havingthe structure expressed by Chemical Formula 1 or Chemical Formula 2 maybe used. The supply amount of the material is in the range from 10 to200 sccm. The inertia carrier gas is He and the supply amount of He isin the range from 300 to 2000 sccm. He does not always need to besupplied as a carrier gas through evaporator 260. Alternatively, part ofHe may be directly supplied to reaction chamber 210. The temperature ofthe substrate is preferably 350 degrees Celsius, the distance betweenthe two electrodes of shower head 205 and substrate heating section 203is preferably 10 mm, the RF power supplied to shower head 205 ispreferably in the range from 50 W to 300 W at a frequency of 13.5 MHz.

The organic siloxane material and carrier gas He are supplied toreaction chamber 210 in a state in which the inner pressure of reactionchamber 210 is maintained in the range from 1.0 to 6.0 Torr.

After He gas is supplied and then the inner pressure of the reactionchamber becomes stable, it is preferable to start supplying thering-shaped organic siloxane material. The desired amount of organicsiloxane material was supplied within around 10 seconds so as to preventthe evaporator from becoming clogged with materials that are polymerizedtherein.

At this point, He was supplied for 500 sccm through the materialevaporator and also directly supplied for 500 sccm to reaction chamber210 through another line. Thereafter, the flow rate of He supplied toreaction chamber 210 was controlled by controlling the flow rate of Hedirectly supplied to reaction chamber 210 through the other line. Theflow rate of He supplied to the evaporator was kept constant at 500sccm. When the flow rate of He supplied through evaporator 260 is keptconstant, the inner temperature of evaporator 260 can become stable andthe material can be stably supplied.

After He and material are stably supplied and the inner pressure of thereaction chamber becomes stable, RF power is applied.

After the growth of the porous film has stopped, while the wafer is leftin the reaction chamber, the supply of organic siloxane material isstopped and only He gas is kept supplied. Thereafter, RF power isapplied so as to cause He plasma to remove carbon from the front surfaceof the porous film and form SiO₂ thereon. Thereafter, reaction chamber210 is repeatedly purged or exhausted and then the wafer is unloadedfrom reaction chamber 210.

Before the film was treated with He plasma, the relative permittivity ofthe film was 25 and the film composition of the material having thestructure expressed by Chemical Formula 5 was Si:O:C=1:1:3.0.

The composition of the porous film can be changed by changing the ratioof the organic siloxane material and He or by changing the structure ofthe material. For example, when the amount of He mixed with 65 sccm oforganic siloxane material was changed from 300 sccm to 1500 sccm, thecomposition ratio of C/Si of the porous film was decreased from 3.4 to2.8. Even if the composition ratio of C/Si is 2.8, the porous filmaccording to this embodiment is still effective.

When the organic siloxane material was changed from a compound havingthe structure expressed by Chemical Formula 2 to a compound having thestructure expressed by Chemical Formula 3, the composition ratio of C/Siof the porous film became 2.1. However, when the ratio of C/Si is toosmall, the density of the film becomes large and metal bridges tend notto be easily formed. Thus, it is preferable that the composition ratioof C be a predetermined value or greater.

Alternatively, as porous film, “Aurora (registered trademark)” may beused. In this case, when the process conditions are adjusted, thecomposition ratio of C/Si can be in the range from 0.4 to 2.0.

When a mixed gas of a gas of siloxane material and oxide gas is usedsuch as “Aurora (registered trademark),” since the first electrode isstrongly oxidized, a valve metal needs to be introduced.

When metal bridges are formed in the porous film, the forming voltagecan be lowered because of the presence of holes formed in the film.However, if hole diameters largely fluctuate, the forming voltagesfluctuate in a wide range. Thus, as a porous film used for the ionconducting layer, the porous film preferably has a single peakdistribution of hole diameters.

It was confirmed that after the porous film is treated with He plasma,the carbon amount decreases on the front surface of the porous film. Inaddition, it was confirmed that the higher the RF power and the longerthe treatment time, the more the carbon amount decreases. Thus, toaccomplish the desired characteristics of the resistance changingelement, it is preferable to appropriately change the processconditions.

Thereafter, second electrode 114 can be formed by depositing a metalfilm of Ru as a target on ion conducting layer 113 by DC sputteringprocess or by long slow sputtering process under the conditions in whichthe DC power is 0.2 kW, the flow rate of Ar gas is 40 sccm, the innerpressure of reaction chamber is 0.27 [Pa], and the substrate temperatureis at room temperature. While the second electrode that becomes theupper electrode is being formed, the metal film is preferably depositedon ion conducting layer 113 at room temperature so as to prevent oxygenfrom being desorbed from ion conducting layer 113.

The resistance changing element that has been formed in such a mannerbecomes a resistance change type nonvolatile element that is a switchingelement that uses migration of copper ions and electrochemical reactionsin the ion conducting member. With a voltage applied to the resistancechanging element or a current that flows in the resistance changingelement, the ON/OFF states of the resistance changing element arecontrolled. For example, based on the electromigration of copper intitanium oxide film 112 and ion conducting layer 113, the ON/OFF statesof the resistance changing element are controlled. The resistancechanging element according to the present invention can be used not onlyfor a switch element, but also for a memory device that is based on bothnonvolatility and resistance change characteristics.

A wafer (not shown) or a substrate (not shown) on which the resistancechanging element is mounted may be a substrate on which a semiconductordevice has been formed. For example, the wafer or substrate may be asilicon substrate, a mono-crystal substrate, an SOI (Silicon onInsulator) substrate, a TFT (Thin Film Transistor) substrate, or asubstrate for a liquid crystal.

According to this embodiment, while copper, of which the lower electrodeis made, is prevented from being oxidized, the ion conducting layer canbe formed and thereby a resistance changing element having highswitching characteristics can be obtained.

When an titanium oxide film is formed by depositing titanium metal oncopper wires and then forming an ion conducting layer by plasma CVDprocess, while the ion conducting layer is being formed, titanium can beoxidized and thereby titanium oxide can be formed.

Third Embodiment

Next, with reference to the accompanying drawings, a semiconductordevice according to a third embodiment of the present invention will bedescribed. According to this embodiment, a resistance changing elementis formed in a multi-layered wiring layer of the semiconductor device.

FIG. 6 is a partial sectional view schematically showing a structure ofthe semiconductor device according to this embodiment.

Resistance changing element 25 of the semiconductor device according tothis embodiment has first wire 5 a that serves as a lower electrode;titanium oxide film 8; ion conducting layer 9; first upper electrode 10;and second upper electrode 11.

In the semiconductor device according to this embodiment, hard mask film23 that is a thick film is formed on a laminate of first upper electrode10, second upper electrode 11, and hard mask film 12. The side surfacesof titanium oxide film 8, ion conducting layer 9, first upper electrode10, second upper electrode 11, hard mask film 12, and hard mask film 23are coated with protective insulation film 24. Protective insulationfilm 24 is not formed on hard mask film 23, but on insulation barrierfilm 7. FIG. 6 also shows wires (5 b, 18 b, and 19 b) that are notelectrically connected to resistance changing element 25. Plug 19 b ofsecond wire 18 b is electrically connected to first wire 5 b throughbarrier metal 20 b. The structure of the resistance changing elementsection according to the third embodiment is the same as that accordingto the first embodiment.

First wire 5 a is a wire buried in a wire groove formed in inter-layerinsulation film 4 and barrier insulation film 3 through barrier metal 6a. First wire 5 a also serves as a lower electrode of resistancechanging element 25 and is directly in contact with titanium oxide film8. An electrode layer or the like may be inserted between first wire 5 aand titanium oxide film 8. When the electrode layer is formed, it ispreferably formed together with titanium oxide film 8 and ion conductinglayer 9. First wire 5 a is made of a metal that can migrate in the ionconducting layer and that can conduct ions, for example, Cu or the like.The front surface of first wire 5 a may be coated with CuSi.

First wire 5 b is a wire that is buried in a wire groove formed ininter-layer insulation film 4 and barrier insulation film 3 throughbarrier metal 6 b. First wire 5 b is not connected to resistancechanging element 25, but electrically connected to plug 19 b throughbarrier metal 20 b. First wire 5 b is made of the same material as firstwire 5 a. The material of first wire 5 b is, for example, Cu.

Barrier metals 6 a and 6 b are conductive films having barriercharacteristics. Barrier metals 6 a and 6 b coat the side and bottomsurfaces of first wires 5 a and 5 b so as to prevent the metal containedin first wires 5 a and 5 b from migrating to inter-layer insulation film4 and the lower layer. When first wires 5 a and 5 b are made of metalswhose main component is Cu, barrier metals 6 a and 6 b are made of ametal having a high melting point, its nitride, or its laminate, forexample, tantalum (Ta), tantalum nitride (TaN), titanium nitride (TiN),or tungsten carbon nitride (WCN).

Second wire 18 a is a wire buried in a wire groove formed in inter-layerinsulation film 17 and etching stopper film 16 through barrier metal 20a. Second wire 18 a is formed together with plug 19 a. Plug 19 a isburied in a prepared hole formed in hard mask film 23 and hard mask film24 through barrier metal 20 a. Plug 19 a is electrically connected tosecond upper electrode 11 through barrier metal 20 a.

Second wire 18 b is a wire buried in a wire groove formed in inter-layerinsulation film 17 and etching stopper film 16 through barrier metal 20b. Second wire 18 b is formed together with plug 19 b. Plug 19 b isburied in a prepared hole formed in inter-layer insulation film 15,protective insulation film 24, and insulation barrier film 7 throughbarrier metal 20 b. Plug 19 b is electrically connected to first wire 5b through barrier metal 20 b. Second wire 18 b and plug 19 b are made ofthe same material as the material of second wire 18 a and plug 19 a. Thematerial of second wire 18 b and plug 19 b is, for example, Cu.

Barrier metals 20 a and 20 b are conductive films having barriercharacteristics. Barrier metals 20 a and 20 b coat the side and bottomsurfaces of second wires 18 a and 18 b and plugs 19 a and 19 b so as toprevent metal contained in second wires 18 a and 18 b (including plugs19 a and 19 b) from migrating to inter-layer insulation films 15 and 17and the lower layer. When second wires 18 a and 18 b and plugs 19 a and19 b are made of metals whose main component is Cu, barrier metals 20 aand 20 b are made of a metal having a high melting point, a nitridethereof, or a laminate thereof, for example, Ta, TaN, TiN, or WCN.

Barrier metals 20 a and 20 b are preferably made of the same material assecond upper electrode 11. When barrier metals 20 a and 20 b have astructure of a laminate of TaN (lower layer)/Ta (upper layer), TaN asthe material of the lower layer is preferably used for the second upperelectrode. Alternatively, when barrier metals 20 a and 20 b have astructure of a laminate of Ti (lower layer)/Ru (upper layer), Ti as thematerial of the lower layer is preferably used for second upperelectrode 11.

Hard mask film 23 is a film that becomes a hard mask that is used whenhard mask film 12 is etched. The material of hard mask film 23 ispreferably different from that of hard mask film 12. For example, whenhard mask film 12 is a SiN film, hard mask film 23 may be a SiO₂ film.

Protective insulation film 24 is an insulation film that serves toprevent oxygen from being desorbed from the ion conducting layer withoutdamaging resistance changing element 25. Protective insulation film 24may be a SiN film, an SiCN film, or the like. Protective insulation film24 is preferably made of the same material as hard mask film 12 andinsulation barrier film 7. When protective insulation film 24 is made ofthe same material as hard mask film 12 and insulation barrier film 7,since protective insulation film 24, insulation barrier film 7, and hardmask film 12 are formed together with each other, the adherence of theinterfaces can be improved.

Fourth Embodiment

Next, with reference to the accompanying drawings, a method formanufacturing the semiconductor device shown in FIG. 6 will be describedas a fourth embodiment of the present invention. FIG. 7A to FIG. 10B aresectional views showing a semiconductor device at each of steps that areused in the method for manufacturing the semiconductor device shown inFIG. 6.

First, inter-layer insulation film 2, barrier insulation film 3, andinter-layer insulation film 4 are successively formed on semiconductorsubstrate 1. In this context, semiconductor substrate 1 may be asemiconductor substrate itself or a substrate on which a semiconductordevice (not shown) has been formed. For example, inter-layer insulationfilm 2 is a silicon oxide film having a film thickness of 300 nm;barrier insulation film 3 is a SiN film having a film thickness of 50nm; and inter-layer insulation film 4 is a silicon oxide film having afilm thickness of 300 nm.

Thereafter, wire grooves are formed in inter-layer insulation film 4,barrier insulation film 3, and inter-layer insulation film 4 by thelithography. The lithography includes a photoresist forming step forforming a resist having a predetermined pattern on inter-layerinsulation film 4; a dry etching step for performing anisotropy etchingon the laminate film with a mask of the resist; and a step for formingwire grooves and then removing the resist from the resultant substrate.

Thereafter, a metal is buried in the wire grooves through barrier metals6 a and 6 b so as to form first wires 5 a and 5 b. Barrier metals 6 aand 6 b have a structure of a laminate of, for example, TaN (filmthickness is 5 nm)/Ta (film thickness is 5 nm). The material of secondwires 5 a and 5 b is, for example, copper. Thereafter, insulationbarrier film 7 is formed on inter-layer insulation film 4 includingfirst wires 5 a and 5 b. Insulation barrier film 7 is, for example, aSiN film having a film thickness of 50 nm.

Thereafter, a hard mask film (not shown) is formed on insulation barrierfilm 7. The hard mask film is, for example, a silicon oxide film.Thereafter, a photoresist having a predetermined opening pattern (notshown) is formed on the hard mask film and then the opening pattern istransferred to the hard mask film with a mask of the photoresist by dryetching. Thereafter, the photoresist is stripped by oxygen plasma ashingand so forth.

Thereafter, insulation barrier film 7 that is exposed to the openingportion of the hard mask film is etched back with a mask of the hardmask film (in this example, by reactive dry etching). As a result, theopening portion that reaches the upper surface of first wire 5 a isformed in insulation barrier film 7. Thereafter, copper oxide formed onthe exposed surface of first wire 5 a is removed by an organic strippingprocess using an amine type resist stripping solution. In addition,etch-back byproduct and so forth are removed (FIG. 7A). The step forforming the structure shown in FIG. 7A is referred to as step B1.

The depth of the wire grooves formed in inter-layer insulation film 4and barrier insulation film 3 at step B1 is the total of the filmthickness of inter-layer insulation film 4 and around 70 nm for whichinter-layer insulation film 4 is over-etched from the lower surface. Thewire grooves piece barrier insulation film 3. Inter-layer insulationfilm 2 is bored to a depth of around 20 nm from the upper surface. Byetching barrier insulation film 3 in such a manner, the release propertyof the wire grooves can be improved.

At step B1, the opening portion of insulation barrier film 7 is formedby reactive dry etching under the conditions in which the gas flow ratesof CF₄/Ar are 25/50 sccm, the inner pressure of the chamber is 0.53[Pa], the source power is 400 W, and the substrate bias power is 90 W.When the source power is decreased or the substrate bias power isincreased, since etching gas can be highly ionized, the taper angle canbe decreased. At this point, when the remaining film depth of theopening portion at the bottom of insulation barrier film 7 is around 30nm, insulation barrier film 7 can be etched for a depth of 55 nm(equivalent to over-etching of around 80%).

At step B1, the substrate may be heated at 350 degrees Celsius in areducing atmosphere. When insulation barrier film 7 is etched back inthe sputtering apparatus, the substrate can be heated in a heat chamberof the sputtering apparatus.

In addition, at step B1, insulation barrier film 7 can be etched back byRF etching using a nonreactive gas, Ar gas, under the conditions inwhich the flow rate of Ar gas is 30 sccm, the inner pressure of thereaction chamber is 1.3 [Pa], the source power is 290 W, and thesubstrate bias power is 130 W. The RF etching time can be quantified bythe etching amount of the SiO₂ film formed by plasma CVD process. Thus,the RF etching time is equivalent to 3 nm of the film thickness of theSiO2 film.

At the beginning of step B1, since first wire 5 b is coated withinsulation barrier film 7, wire 5 b except for the opening portion isetched by RF etching.

Thereafter, a Ti metal film having a film thickness of 2 nm is depositedon insulation barrier film 7 including first wires 5 a and 5 b by DCsputtering process. Thereafter, a porous film (that is manufactured fromthe material having the structure expressed by Chemical Formula 2 andthat has a film thickness of 7 nm) on insulation barrier film 7including first wires 5 a and 5b by RF plasma process. At this point,the Ti metal film is fully oxidized by oxygen plasma that is generatedby the decomposition of the material of the porous film and therebytitanium oxide film 8 is obtained. Thereafter, first upper electrode 10and second upper electrode 11 are successively formed on the porous film(FIG. 7B). First upper electrode 10 is, for example, Ru having a filmthickness of 10 nm. Second upper electrode 11 is, for example, Ta havinga film thickness of 50 nm. The step that forms the structure shown inFIG. 7B from the structure shown in FIG. 7A is referred to as step B2.

At step B2, the porous film can be deposited by plasma CVD process underthe conditions in which RF power is 500 to 300 W, the temperature is 350degrees Celsius, a mixed gas of He is used, and the inner pressure ofthe reaction chamber is 1.0 to 6.0 [Torr].

Specifically, the porous film can be formed in an 8-inch plasma CVDreactor under the conditions in which the flow rate of He gas is 1500sccm, the inner pressure of the reactor is 3.5 [Torr], and the RF poweris 100 W. Under these conditions, observation results using across-sectional TEM (Transmission Electron Microscope) confirmed thatwhen Ti having a film thickness of 2 nm was deposited, titanium oxidehaving a film thickness of 3.0 nm was formed.

Depending on the specifications of the apparatus, if the oxidizing powerof oxygen plasma is strong, by increasing the film thickness of Ti, thefirst electrode can be prevented from being oxidized.

However, a Ti film does not always need to be formed on insulationbarrier film 7. Alternatively, the RF power may be lowered or the flowrate of the material may be increased. As a result, since the materialis prevented from being decomposed and oxygen plasma is prevented frombeing generated, copper can be prevented from being oxidized.

After the porous film has been formed, it can be treated with He plasmafor 30 seconds, for example, under the conditions in which the flow rateof He gas is 1500 sccm, the inner pressure of the reaction chamber is2.7 [Torr], and the RF power is 200 W. This treatment result observed byan XPS (X-ray Photoelectron Spectroscopy) confirmed that the compositionratio of C/S on the front surface of the porous film was decreased from2.61 to 2.58.

At step B2, Ru as a target can be deposited on first upper electrode 10by the DC sputtering process under the conditions in which the DC poweris 0.2 kW, Ar gas is used as inertia gas, and the inner pressure of thereaction chamber is 0.27 [Pa]. Likewise, Ta as a target can be depositedon second upper electrode 11 by DC sputtering process under the sameconditions. Since first upper electrodes 10 and 11 are deposited underthe reduced pressure at room temperature, they prevent oxygen from beingdesorbed from conducting layer 9.

At the beginning of step B2, first wire 5 b is covered with insulationbarrier film 7, titanium oxide film 8, ion conducting layer 9, firstupper electrode 10, and second upper electrode 11.

Thereafter, hard mask film 12 and hard mask film 23 are successivelydeposited on second upper electrode 11 (FIG. 7C). Hard mask film 12 is,for example, a SiN film having a film thickness of 30 nm. Hard mask film23 is, for example, a SiO₂ film having a film thickness of 200 nm. Thestep that forms the structure shown in FIG. 7C from the structure shownin FIG. 7B is referred to as step B3.

Hard mask film 12 and hard mask film 23 can be formed by plasma CVDprocess. Hard mask film 12 and hard mask film 23 can be formed by plasmaCVD process, which is common in the field of the related art. The filmgrowth temperature can be selected in a range from 200 degrees Celsiusto 400 degrees Celsius. In this example, the film growth temperature was200 degrees Celsius.

At the beginning of step B2, first wire 5 b is coated with insulationbarrier film 7, titanium oxide film 8, ion conducting layer 9, firstupper electrode 10, second upper electrode 11, hard mask film 12, andhard mask film 23.

Thereafter, a photoresist (not shown) is formed on hard mask film 23 soas to pattern the resistance changing element section. Thereafter, hardmask film 23 is dry-etched with a mask of the photoresist until hardmask film 12 is exposed. Thereafter, the photoresist is removed byoxygen plasma aching and organic stripping. Thereafter, hard mask film12, second upper electrode 11, first upper electrode 10, and ionconducting layer 3 are successively dry-etched with a mask of hard maskfilm 23 (FIG. 8A). The step that forms the structure shown in FIG. 8Afrom the structure shown in FIG. 7C is referred to as step B4.

At step B4, it is preferable that hard mask film 23 be dry-etched suchthat hard mask film 12 is not fully dry-etched. In this case, since ionconducting layer 9 is coated with hard mask film 12, ion conductinglayer 9 is not exposed to oxygen plasma. Likewise, since Ru of firstupper electrode 10 is not exposed to oxygen plasma, first upperelectrode 10 can be prevented from being side-etched. Hard mask film 23can be dry-etched by an ordinary planar dry-etching apparatus.

At step B4, hard mask film 12, second upper electrode 11, first upperelectrode 10, ion conducting layer 9, and titanium oxide film 8 can beetched by a planar dry-etching apparatus. Hard mask film 12 (forexample, a SiN film) can be etched under conditions in which the gasflow rates of CF4/Ar are 25/50 sccm, the inner pressure of the reactionchamber is 0.53 [Pa], the source power is 400 W, and the substrate biaspower is 90 W.

Second upper electrode 11 (for example, Ta) can be etched underconditions in which the flow rate of Cl₂ gas is 50 sccm, the innerpressure of the reaction chamber is 0.53 [Pa], the source power is 400W, and the substrate bias power is 60 W.

At this point, although dispersants caused by etched Ta may adhere tothe side walls of hard mask film 12, second wire 5 a, that also servesas the lower electrode, is electrically separated from the side walls ofhard mask film 12 through insulation barrier film 7. Thus, a problemsuch as a short-circuit between second upper electrode 11 and secondwire 5 a does not occur.

In addition, first upper electrode 10 (for example, Ru) can be etchedunder conditions in which the gas flow rates of Cl₂/O₂ are 5/40 sccm,the inner pressure of the reaction chamber is 0.53 [Pa], the sourcepower is 900 W, and the substrate bias power is 100 W.

On the other hand, ion conducting layer 9 (that is manufactured from thematerial having the structure expressed by Chemical Formula 5 and thathas a film thickness of 7 nm) can be etched under conditions in whichthe gas flow rates of Cl₂/CF₄/Ar are 45/15/15 sccm, the inner pressureof the reaction chamber is 1.3 [Pa], the source power is 800 W, and thesubstrate bias power is 60 W. In particular, when chlorine gas isintentionally used, ion conducting layer 9 can be treated while theetching selective ratio between the ion conducting layer 9 and SiN ofthe lower layer is kept high and sub trenches and so forth can beprevented from occurring. At this point, the residual film thickness ofinsulation barrier film 7 on first wires 5 a and 5 b can be adjusted to20 to 40 nm.

Thereafter, protective insulation film 24 is deposited on hard mask film23, hard mask film 12, second upper electrode 11, first upper electrode10, ion conducting layer 9, titanium oxide film 8, and insulationbarrier film 7 (FIG. 8B). Protective insulation film 24 is, for example,a SiN film having a film thickness of 30 nm. The step for forming thestructure shown in FIG. 8B from the structure shown in FIG. 8A isreferred to as step B5.

At step B5, protective insulation film 24 can be formed from highdensity plasma with material gases of SiH_(4 and N) ₂ at a substratetemperature of 200 degrees Celsius. Since a reduction gas such asNH_(3 or H) ₂ is not used, before the film is formed, when the materialgases become stable, a water absorption component can be desorbed fromion conducting layer 9. At this point, when insulation barrier film 7,protective insulation film 24, and hard mask film 12 formed on firstwiring 5 are made of the same material which is SiN film, theyintegrally protect the neighborhood of the resistance changing element.As a result, since the adherence of the interfaces is improved andmoisture absorption, water resistance, and oxygen desorption resistanceare improved, the yield and reliability of the device can be improved.

Thereafter, inter-layer insulation film 15 is deposited on protectiveinsulation film 24 by plasma CVD process (FIG. 8C). Inter-layerinsulation film 15 is, for example, a silicon oxide film having a filmthickness of 500 nm. The step for forming the structure shown in FIG. 8Cfrom the structure shown in FIG. 8B is referred to as step B6.

Thereafter, inter-layer insulation film 15 is flattened by CMP (FIG.9A). The step for flattening inter-layer insulation film 15 is referredto as step B7.

When inter-layer insulation film 15 is flattened, inter-layer insulationfilm 15 can be abraded from the front surface for around 35 nm such thatthe remaining film thickness becomes about 150 nm. At this point,inter-layer insulation film 15 can be abraded by CMP with ordinarycolloidal silica or ceria slurry. According to working example 2, sinceinter-layer insulation film 15 is flattened and hard mask film 23 isexposed, hard mask film 23 and protective insulation film 24 areflattened.

Thereafter, etching stopper film 16 (for example a SiN film having afilm thickness of 50 nm) and inter-layer insulation film 17 (for examplea silicon oxide film having a film thickness of 300 nm) are successivelydeposited on inter-layer insulation film 15 including hard mask film 23and protective insulation film 24 (FIG. 9B). Etching stopper film 16 isfor example a SiN film having a film thickness of 50 nm. Inter-layerinsulation film 17 is, for example, a silicon oxide film having a filmthickness of 300 nm. The step that forms the structure shown in FIG. 9Bfrom the structure shown in FIG. 9A is referred to as step B8.

At step B8, etching stopper film 16 and inter-layer insulation film 17can be deposited by plasma CVD process.

Thereafter, second wires 18 a and 18 b and plugs 19 a and 19 b shown inFIG. 6 are formed by the Via First method of the Dual damascenetechnique.

In the Via First method, a photoresist (not shown) is formed oninter-layer insulation film 17 so as to form prepared holes 71 a and 71b for plugs 19 a and 19 b shown in FIG. 6. Thereafter, inter-layerinsulation film 17 is dry-etched with a mask of the photoresist andthereby prepared hole 71 a for plug 19 a shown in FIG. 6 is formed ininter-layer insulation film 17, etching stopper film 16, and hard maskfilm 23. At the same time, prepared hole 71 b for plug 19 b shown inFIG. 6 is formed in inter-layer insulation film 17, etching stopper film16, and inter-layer insulation film 15. Thereafter, the photoresist isremoved by oxygen plasma ashing and organic stripping (FIG. 10A). Thestep that forms the structure shown in FIG. 10A from the structure shownin FIG. 9B is referred to as step B9.

At step B9, the etching condition and time are adjusted in the dryingetching process such that hard mask film 12 at the bottom of preparedhole 71 a and protective insulation film 24 at the bottom of preparedhole 71 b are not fully dry-etched. At this point, since hard mask film12 and inter-layer insulation film 4 at the bottoms of prepared holes 71a and 71 b are not fully dry-etched, they may be patterned with otherrecycles and formed under different dry etching conditions.

Thereafter, a photoresist (not shown) is formed so as to form wiregrooves 72 a and 72 b for second wires 18 a and 18 b shown in FIG. 6.Thereafter, inter-layer insulation film 17 is dry-etched with a mask ofthe photoresist so as to form wire grooves 72 a and 72 b for secondwires 18 a and 18 b shown in FIG. 6 on inter-layer insulation film 17and etching stopper film 16. Thereafter, the photoresist is removed byoxygen plasma ashing and organic stripping (FIG. 10B). The step forforming the structure shown in FIG. 10B from the structure shown in FIG.10A is referred to as step B10.

When an ARC (Anti-Reflection Coating) is buried at the bottoms ofprepared holes 71 a and 71 b at step B 10, their bottoms can beprevented from opening up.

At step B10, since the bottoms of prepared holes 71 a and 71 b areprotected by hard mask film 12 and protective insulation film 24,prepared holes 71 a and 71 b are not damaged by oxygen in oxygen plasmaashing.

Thereafter, hard mask film 12 at the bottom of prepared hole 71 a isetched and then protective insulation film 24 and insulation barrierfilm 7 at the bottom of prepared hole 71 b are etched so as to exposesecond upper electrode 11 from prepared hole 71 a and first wire 5 bfrom prepared hole 71 b. Thereafter, second wires 18 a and 18 b (forexample, Cu) and plugs 19 a and 19 b (for example, Cu) aresimultaneously formed in wire grooves 72 a and 72 b and prepared holes71 a and 71 b through barrier metals 20 a and 20 b (for example, Tahaving a film thickness of 5 nm). Thereafter, insulation barrier film 21(for example, a SiN film) is deposited on inter-layer insulation film 17containing second wires 18 a and 18 b so as to form the structure shownin FIG. 6. The step that forms the structure shown in FIG. 6 from thestructure shown in FIG. 10B is referred to as step B11.

At step B11, second wires 18 a and 18 b can be formed by the similarprocess to the process used for the wires in the lower layer. At thispoint, the bottom diameter of plug 19 a is preferably smaller than thediameter of the opening portion of insulation barrier film 7. Accordingto this embodiment, the diameter of the bottom of plug 19 a is, forexample, 240 nm and the diameter of the opening portion of insulationbarrier film 7 is 400 nm. The width of first wire 5 a that also servesas the lower electrode of resistance changing element 25 is preferablygreater than the diameter of the opening portion of insulation barrierfilm 7. In addition, when barrier metal 20 a and second upper electrode11 are made of the same material, the contact resistance between plug 19and second upper electrode 11 can be decreased and the elementperformance can be improved (the resistance of resistance changingelement 25 can be decreased when the element is in the ON state).

Working Example 1

In this working example, the structure and electric characteristics ofthe resistance changing element manufactured by the method according tothe fourth embodiment will be described. As the material of organicsilica, the material having the structure expressed by Chemical Formula5 was used.

The insulation film as the ion conducting layer was formed under theconditions in which the flow rate of material was 40 sccm, the flow rateof He gas was 1500 sccm, the inner pressure of the reaction chamber is3.5 [Torr], and the RF power was 88 W. The plasma applying time was setsuch that the film thickness of the grown insulation film became 6 nm.

The composition ratio of the formed insulation film was Si:O:C=1:1:2.7and the relative permittivity was 2.5. A measurement result for theinsulation film by XRR confirmed that the film was a porous film thathad a single peak distribution of hole diameters and the average holediameter was 0.35 nm.

The composition ratio of the insulation film may be changed such thatthe resistance changing element has desired operational characteristics.For example, when the supply amount of the material is decreased, theflow rate of He gas is increased, and when the RF power is increased,the amount of C can be decreased while the composition ratio of Si/O iskept in the range from 2.0 to 3.0 of the composition ratio of C/Si. Inaddition, when the composition ratio of the material of organic silicais changed, the composition ratio of the film can be adequately changed.

A cross-sectional TEM observation of the resistance changing elementformed by the method according to the fourth embodiment confirmed that atitanium oxide film, a porous film, and a SiO₂ film were directly formedon the upper surface of copper.

FIG. 11 is a graph showing I-V characteristics of the resistancechanging element manufactured by the method according to the fourthembodiment. The horizontal axis of the graph represents voltages appliedbetween the first electrode and the second electrode, whereas thevertical axis represents currents that flow between the electrodes. TheI-V characteristics obtained when the resistance changing element isinitially operated are denoted by black circles, whereas the I-Vcharacteristics obtained when the resistance changing element isoperated second time are denoted by white squares.

It was confirmed that when the voltage applied between the first andsecond electrodes was increased in the plus direction, forming tookplace at around 3 V and the porous film changed from the high resistancestate to the low resistance state (around 100 Ω). It was also confirmedthat after the porous film had changed to the high resistance state,when a voltage of 3 V or lower was applied in the reverse direction, theporous film changed from the high resistance state to the low resistancestate. It was also confirmed that when the resistance changing elementwas operated a second time, the porous film changed to the highresistance state nearly at the same voltage as the first voltage(forming voltage).

FIG. 12 is a graph showing dielectric breakdown voltages in which aminus voltage applied to the resistance changing element formed by themethod according to the fourth embodiment is increased. The horizontalaxis of the graph represents film thicknesses, whereas the vertical axisrepresents dielectric breakdown voltages. Measurement values for aTa₂O_(5 film and a Ta) _(0.8)Si_(0.2)O_(x) film are also plotted on thegraph so as to compare the porous film with these films. Measurementvalues for the porous film (pSiOC Vset) are denoted by black circles;measurement values for the Ta₂O₅ film (TaO Vset) are denoted by whitesquares; and measurement values for the Ta_(0.8)Si_(0.2)O_(x) film(TaSiO Vset) are denoted by black diamonds.

Regardless of the material, as the film thickness of the ion conductinglayer increases, the effective electric field decreases. Thus, thedielectric strength voltage tends to increase. When the porous film andother materials are compared, it is clear that the dielectric strengthvoltage of the porous film is greater than those of the other materials.This means that when metal bridges are formed in the porous film, sincethe ion conducting layer is less damaged compared to the othermaterials, the insulation reliability improves.

In other words, when metal bridges are formed in the ion conductinglayer, if it is a porous film, it has holes for metal ions. Thus, whenions that serve as metal bridges exit from or enter the ion conductinglayer, it is less damaged.

Next, a memory device that has memory cells provided with a resistancechanging elements according to this working example will be described.FIG. 13A and FIG. 13B are schematic diagrams showing an example of thestructure of a memory device using resistance changing elementsaccording to this working example. FIG. 13A shows principal sections ofthe memory device, whereas FIG. 13B shows an example of the structure ofa memory cell.

As shown in FIG. 13A, the memory device has memory cell array 500,column address circuit 510, and row address circuit 520. Column addresscircuit 510 has 5-bit decoder 511 and level shifter 512. Row addresscircuit 520 has 5-bit decoder 521 and level shifter 522.

Memory cell array 500 is provided with a plurality of memory cells 530shown in FIG. 13B. Memory cell 530 has resistance changing element 531and transistor 533. A drain electrode of transistor 533 is connected toa bit line (BL) and a source electrode is connected to resistancechanging element 531. The material of BL is copper.

In addition, memory cell array 500 is provided with transistor 543 whosesource electrode is connected to a plate line (PL). One of twoelectrodes of resistance changing element 531 is connected to transistor533, whereas the other electrode is connected to the drain electrode oftransistor 543.

The gate width W of each of transistors 533 and 543 is 3 μm and thesource drain current that flows in the ON state at VDD=5 V is 1 mA. Agate electrode of transistor 543 is connected to level shifter 522 ofrow address circuit 520. A gate electrode of transistor 533 is connectedto level shifter 512 of column address circuit 510.

When an address signal is input from the outside to column addresscircuit 510 and row address circuit 520, one of memory cells 530 isselected from memory cell array 500 so as to change the state ofresistance changing element 531 of selected memory cell 530 and measurethe resistance value of resistance changing element 531.

Specifically, when a plus voltage is applied to the BL connected totransistor 533 of selected memory cell 530 and a pulse voltage isapplied to the gate electrode of transistor 533, the resistance ofresistance changing element 531 can be changed.

Next, measurement results of resistance changes of resistance changingelements of the memory cell array shown in FIG. 13A will be described.FIG. 14 is a graph showing the distribution of resistance values of eachelement of the memory cell array shown in FIG. 13A. The horizontal axisof the graph represents resistance values of resistance changingelements.

FIG. 14 shows the distribution of resistance values of each 1-k bitresistance changing element when a forming voltage of 4 V is applied andpulses are supplied to the resistance changing element at a period of 1μsec and the ON/OFF operations are repeated. In FIG. 14, Rini representsthe resistance value of the resistance changing element in the initialstate before pulses are applied; Ron represents the resistance value inthe ON state of the resistance changing element; and Roff represents theresistance value in the OFF state of the resistance changing element.FIG. 14 reveals that the resistance value in the OFF state of theresistance changing element is around 10^(7Ω) and that the resistancevalue in the ON state of the resistance changing element is around10^(3Ω.) The resistance value in the OFF state of the resistancechanging element is around 10⁴ times greater than that in the ON stateof the resistance changing element. This measurement result confirmedthat the resistance changing operation of the resistance changingelement can be accomplished with high yield at a forming voltage of 4 Vor lower.

Working Example 2

Individual organic siloxane materials having the structures expressed byChemical Formula 3 to Chemical Formula 9 were evaluated as ionconducting layers. Resistance changing elements were formed by themethod according to the fourth embodiment. To prevent redundancy, adetailed description will be omitted.

The supply amount of the material was 40 sccm regardless of the type. Asother process conditions, the flow rate of He gas was 1500 sccm, theinner pressure of the reaction chamber was 3.5 [Torr], and the RF powerwas 88 W. Since the growth speed of the insulation film depends on thetype, the amount time required for applying plasma was controlled suchthat the film thickness of the grown insulation film became 6 nm. Whenthe material having the structure expressed by Chemical Formula 6 orChemical Formula 8 was used, since the growth speed remarkablydecreased, the RF power was increased to 150 W.

The relative permittivities of the films formed from the materialshaving the structures expressed by Chemical Formula 3 to ChemicalFormula 9 were k=2.7, 2.6, 2.5, 2.9, 2.7, 3.2, 2.8, respectively. Therelative permittivity of the material having a side chain of a vinylgroup was low. The relative permittivity of the material having asix-membered ring was also low. It was confirmed that the materialhaving the structure expressed by Chemical Formula 5 had the highestcarbon content and thereby its relative permittivity was low. Sweepmeasurements confirmed that with the ion conducting layers made of thesematerials, the ON/OFF operations were performed at a forming voltage of5V or less. It was revealed that when these materials were formed byplasma CVD process, the relative permittivities of the films were in therange from 2.5 to 3.5 and the switching characteristics were excellent.This means that the lower the density of the film, the easier the ionsmigrate.

Measurement results for a distribution of hole diameters of films formedfrom these materials by an XRR confirmed that the individual films had asingle peak distribution of hole diameters. The distribution of holediameters corresponds to the distribution of forming voltages. Thus, ifthere are a plurality of peaks in the distribution of hole diameters,forming voltages fluctuate in a wide range. Thus, to prevent operationvoltages from fluctuating, each film preferably has a single peakdistribution of hole diameters.

Fifth Embodiment

A resistance changing element according to a fifth embodiment of thepresent invention has ruthenium (Ru) as a first electrode, TiO₂ as aresistance changing layer, an insulation film as a buffer layer, and Ruas a second electrode that are successively formed. The resistancechanging element according to this embodiment has a structure in whichporous film 102 shown in FIG. 3 is substituted with a resistancechanging layer; and modified layer 117 is substituted with a bufferlayer. A sectional view of the resistance changing element according tothis embodiment is omitted. In the following, a method that forms theresistance changing element will be described focused on the differencebetween the structure according to this embodiment and the structureshown in FIG. 3.

A laminate electrode having TaN (lower layer)/Ru (upper layer)=5 nm/5 nmis formed as a lower electrode on a silicon substrate having a lowresistance by PVD (Physical Vapor Deposition) process. Thereafter, a Tifilm having a film thickness of 2 nm is formed on the lower electrode byPVD process. Thereafter, an insulation film that has a film thickness of6 nm and that is made of SiOC is formed on the Ti film by plasma CVDprocess described in the second embodiment. At this point, oxygen plasmagenerated by the decomposition of the material, while the insulationfilm made of SiOC is grown, oxidizes the Ti film and thereby TiO₂ isformed. The resultant TiO₂ serves as a resistance changing layer. Inaddition, Ru (lower layer)/Ta (upper layer)=10 nm/50 nm as materials ofthe upper electrode are formed on the insulation film. The laminate filmof Ru/Ta is patterned with a mask of a stencil so as to form an upperelectrode. The size of the plane pattern of the upper electrode was 20μm².

Since the conditions under which the SiO₂ film is formed are the same asthose in the method according to the first embodiment, their detaileddescription will be omitted.

TiO₂ is known as a material whose resistance value changes correspondingto a voltage applied thereto as oxygen holes occur. However, if the filmthickness of TiO₂ is small, since a leak current flows between the lowerelectrode and the insulation layer, when a SiOC layer is used as abuffer layer, the switching voltage can be set to a desired voltage. Toconfirm this phenomenon, a laminate film of a resistance changing layerand a buffer layer was manufactured and a prober was directly broughtinto contact with the laminate film so as to measure resistance changingcharacteristics. It was confirmed that when a voltage of around 3 V wasapplied to the laminate film, it changed to the low resistance state andwhen a voltage of around −1 V was applied to the laminate film, itchanged to the high resistance state. Thus, it was confirmed that anON/OFF ratio of around 3 digits was obtained.

In this embodiment, the case in which a TiO₂ layer was used for aresistance changing layer was described. Alternatively, another filmsuch as NiO that is known as a resistance changing layer may be used.Further alternatively, the upper electrode and the lower electrode maybe made of a metal compound such as TiN.

Although the present invention has been described with reference topreferred embodiments, they are just examples and thereby they do notlimit the present invention.

For example, technologies for fabricating devices for CMOS circuits inthe field based on which the invention was made by the inventors of thepresent invention have been described. In addition, examples in which aresistance changing element is formed on copper wires formed on asemiconductor substrate have been described. However, the presentinvention is not limited to such examples. It should be appreciated thatthe present invention can be applied to semiconductor products having amemory circuit such as a DRAM (Dynamic RAM), an SRAM (Static RAM), aflash memory, an FRAM (Ferro Electric RAM), an MRAM (Magnetic RAM), aresistance changing memory, and a bipolar transistor; semiconductorproducts having a logical circuit such as a microprocessor; and copperwires of boards and packages that mount both of them.

In addition, the present invention can be applied to the bonding of anelectronic circuit device, an optical circuit device, a quantizingcircuit device, a micro-machine, an MEMS (Micro Electro MechanicalSystem) for a semiconductor device. Moreover, although embodiments ofswitch functions have been described, the present invention can beapplied to memory devices that use both nonvolatile and resistancechanging characteristics.

In addition, the bonding method for a substrate according to the presentinvention can be confirmed from the manufactured state. Specifically,when the cross section of a device is observed by a TEM, a lowerelectrode, an ion conducting layer, and an upper electrode made ofcopper can be confirmed. Moreover, it can be confirmed thatmulti-layered wiring is made of copper. When a resistance changingelement is mounted, it can be confirmed that in the state in which thelower surface of the resistance changing element is a copper wire and italso serves for a lower electrode, an ion conducting layer is present.Moreover, by analyzing the composition of the ion conducting layer byTEM, EDX (Energy Dispersive X-ray Spectroscopy), EELS (ElectronEnergy-Loss Spectroscopy) or the like, the materials that were used canbe confirmed. Specifically, it can be determined whether or not an ionconducting layer formed on a copper wire is a film containing bothoxygen and carbon. In addition, when the upper electrode that is incontact with the ion conducting layer is made of Ru, it can bedetermined that the structure according to the present invention isused.

As an effect of the present invention, when a porous film is used for aresistance changing layer, the forming voltage for the resistancechanging element can be lowered while high insulation reliability ismaintained and thereby the resistance changing element can be operatedat a low voltage.

While the invention has been particularly shown and described withreference to exemplary embodiments thereof, the invention is not limitedto these embodiments. It will be understood by those of ordinary skillin the art that various changes in form and details may be made thereinwithout departing from the spirit and scope of the present invention asdefined by the claims.

This application is based upon and claims the benefit of priority fromJapanese Patent Application No. 2009-258007 filed on Nov. 11, 2009, thecontent of which is incorporated by reference.

REFERENCE SIGNS LIST

101, 110 First electrodes

102 Porous film

103, 114 Second electrodes

112 Titanium oxide film

113 Ion conducting layer

500 Memory cell array

530 Memory cell

531 Resistance changing element

533, 543 Transistors

1. A resistance changing element comprising: a first electrode and asecond electrode; and an ion conducting layer that is formed betweensaid first electrode and said second electrode and that contains atleast oxygen and carbon.
 2. The resistance changing element according toclaim 1, wherein said ion conducting layer is a film that contains atleast silicon as well as said oxygen and said carbon.
 3. The resistancechanging element according to claim 1, wherein said ion conducting layeris a porous film.
 4. The resistance changing element according to claim2, wherein said ion conducting layer is a film that contains asix-membered ring-shaped or an eight-membered ring-shaped siloxanestructure.
 5. The resistance changing element according to claim 1,wherein said ion conducting layer is a film that contains at last anunsaturated hydrocarbon group.
 6. The resistance changing elementaccording to claim 3, wherein said ion conducting layer has a relativepermittivity of 2.5 to 3.5 and a single peak distribution of holediameters.
 7. The resistance changing element according to claim 1,wherein said first electrode is made of copper and said second electrodeis made of ruthenium.
 8. The resistance changing element according toclaim 1, wherein a layer whose carbon content is smaller than that ofthe ion conducting layer is formed between said ion conducting layer andsaid second electrode.
 9. The resistance changing element according toclaim 1, wherein an oxide of a valve metal is formed between said firstelectrode and said ion conducting layer.
 10. A semiconductor devicecomprising: a resistance changing element according to claim
 1. 11. Amethod for forming a resistance changing element, comprising: forming aninsulation film by plasma CVD process with a gas in which a gas of anorganic silica compound having a skeleton of at least silicon and oxygenis diluted with an inertia gas; and forming a second electrode on saidinsulation film.
 12. The method for forming a resistance changingelement according to claim 11, wherein said insulation film forming stepis performed by forming said insulation film without an oxidation gas.13. The method for forming a resistance changing element according toclaim 11, wherein said organic silica compound is a ring-shaped organicsilica compound having a skeleton of silicon and oxygen.
 14. The methodfor forming a resistance changing element according to claim 11, whereinsaid organic silica compound is a ring-shaped organic silica compoundthat has a skeleton of silicon and oxygen and whose side ring is boundto at least one unsaturated hydrocarbon group.
 15. The method forforming a resistance changing element according to claim 11, whereinsaid organic silica compound has a structure expressed by any one ofChemical Formulas that follow and R1 and R2 each is any one from amonghydrogen, methyl group, ethyl group, propyl group, isopropyl group, andvinyl group.


16. The method for forming a resistance changing element according toclaim 15, wherein said organic silica compound has a structure expressedby any one of Chemical Formulas that follow.


17. The method for forming a resistance changing element according toclaim 15, wherein said organic silica compound has a structure expressedby any one of Chemical Formulas that follow.


18. The method for forming a resistance changing element according toclaim 11, wherein the step that forms said insulation film is precededby forming a valve metal film on the first electrode and forming theinsulation film, said step including oxidizing a metal contained in thevalve metal film.
 19. The method for forming a resistance changingelement according to claim 18, wherein said first electrode is made ofcopper, said valve metal is made of titanium, and said second electrodeis made of ruthenium.
 20. The method for forming a resistance changingelement according to claim 11, further comprising: a step that radiatesinertia gas plasma to a front surface of the porous film, said stepbeing performed between the step that forms said insulation film and thestep that forms said second electrode.